Multi-piece solid golf ball

ABSTRACT

The present invention provides a multi-piece solid golf ball having a core, an envelope layer encasing the core, an intermediate layer encasing the envelope layer, and a cover which encases the intermediate layer and has formed on a surface thereof a plurality of dimples. The core is composed overall of an inner layer and an outer layer which are each formed primarily of a rubber material, with the outer core layer being harder than the inner core layer. The envelope layer, intermediate layer and cover have respective thicknesses which satisfy the condition: cover thickness&lt;intermediate layer thickness&lt;envelope layer thickness, and have respective material hardnesses (Shore D hardness) which satisfy the condition: envelope layer material hardness&lt;intermediate layer material hardness&gt;cover material hardness. The golf ball has a lower spin rate on full shots with a driver, further increasing the distance traveled by the ball. Moreover, it has a good controllability, maintaining in particular a straight trajectory on full shots, and also has an excellent scuff resistance.

BACKGROUND OF THE INVENTION

The present invention relates to a multi-piece solid golf ball composedof a core, an envelope layer, an intermediate layer and a cover thathave been formed as successive layers. More specifically, the inventionrelates to a multi-piece solid golf ball which has a satisfactory flightperformance and controllability when used by professionals and otherskilled golfers, is able in particular to maintain a straight trajectoryon full shots, and has an excellent scuff resistance.

A variety of golf balls have hitherto been developed for professionalsand other skilled golfers. Of these, multi-piece solid golf balls inwhich the hardness relationships among layers encasing the core, such asan intermediate layer and a cover layer, have been optimized are in wideuse because they achieve both a superior distance in the high head speedrange and good controllability on shots taken with an iron and onapproach shots. Another important concern is the proper selection ofthicknesses and hardnesses for the respective layers of the golf ball inorder to optimize flight performance, the feel of the ball when played,and the spin rate of the ball after being struck with a club,particularly given the large influence of the spin rate on control ofthe ball. A further key concern in ball development, arising from thedesire that golf balls also have durability under repeated impact andsuppress burr formation on the ball surface (have improved scuffresistance) when repeatedly played with different types of clubs, is howbest to protect the ball from external factors.

The three-piece solid golf balls having an outer cover layer formedprimarily of a thermoplastic polyurethane that are disclosed in, forexample, JP-A 2003-190330, JP-A 2004-049913, JP-A 2004-97802 and JP-A2005-319287 were intended to meet such needs. However, these golf ballsfail to achieve a sufficiently low spin rate when hit with a driver;professionals and other skilled golfers desire a ball which delivers aneven longer distance.

Meanwhile, efforts to improve the flight and other performancecharacteristics of golf balls have led to the development of ballshaving a four-layer construction, i.e., a core enclosed by threeintermediate and cover layers, that allows the ball construction to bevaried among the several layers at the interior. Such golf balls havebeen disclosed in, for example, JP-A 9-248351, JP-A 10-127818, JP-A10-127819, JP-A 10-295852, JP-A 10-328325, JP-A 10-328326, JP-A10-328327, JP-A 10-328328, JP-A 11-4916 and JP-A 2004-180822.

Yet, as golf balls for the skilled golfer, such balls have a poorbalance of distance and controllability or fall short in terms ofachieving a lower spin rate on shots with a driver, thus limiting thedegree to which the total distance can be increased.

Moreover, in the multi-piece solid golf ball described in U.S. Pat. No.6,994,638, the thickness and hardness relationships among the respectivelayers such as the intermediate layer and the cover are not disclosed.This ball is thus inadequate for achieving the spin rate-lowering effecton shots with a driver that is desired in a golf ball for the skilledgolfer.

Each of the golf balls disclosed in JP-A 2001-17569, U.S. Pat. No.6,416,425 and JP-A 2001-37914 is a multi-piece solid golf ball of fiveor more layers in which the four or more layers encasing the core, suchas envelope layers and cover layers, have various hardnessrelationships. Yet, owing to the fact that the envelope layers are madeof resin materials and to the differing hardness relationships andthickness relationships among the respective layers, such balls fail toachieve the performance needed in a golf ball for skilled players.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a golfball which has a satisfactory flight performance and controllabilitywhen used by professionals and other skilled golfers, is able inparticular to maintain a straight trajectory on full shots, and has anexcellent scuff resistance.

The present invention provides, as the basic construction in a golf balldesign, a multilayer structure having a two-layer core composed of aninner core layer and an outer core layer, and having three or more outerlayers (envelope layer/intermediate layer/cover) enclosing the core.Moreover, in the invention, by forming the two-layer core so that theouter core layer is harder than the inner core layer, by adjusting thematerial hardnesses in the envelope layer/intermediate layer/coverconstruction so as to impart a hardness relationship therebetween,expressed in the order of the successive layers, of soft/hard/soft, andby also optimizing the layer thickness relationships in the envelopelayer/intermediate layer/cover construction, it was possible through thesynergistic effects of these hardness relationships and layer thicknessrelationships to resolve the above-described problems encountered in theprior art. That is, the golf ball of the invention, when used byprofessionals and other skilled golfers, provides a fully satisfactoryflight performance and controllability. In particular, on full shotswith an iron, a lower spin rate is achieved, enabling the ball to travela longer distance. At the same time, the ball exhibits sufficientcontrollability in the short game. The ball also has an excellent scuffresistance. Such a combination of effects was entirely unanticipated.The inventor, having thus found that the technical challenges recitedabove can be overcome by the foregoing arrangement, ultimately arrivedat the present invention.

Compared with the invention recited in the related application Ser. No.11/443,130 previously filed by the inventor, the golf ball of thepresent invention has a lower spin rate on full shots with an iron, thusincreasing the distance of travel, and is also better able to follow astraight trajectory.

Accordingly, the invention provides the following multi-piece solid golfballs.

[1] A multi-piece solid golf ball comprising a core, an envelope layerencasing the core, an intermediate layer encasing the envelope layer,and a cover which encases the intermediate layer and has formed on asurface thereof a plurality of dimples, wherein the core is composedoverall of an inner layer and an outer layer which are each formedprimarily of a rubber material, the outer core layer being harder thanthe inner core layer; and the envelope layer, intermediate layer andcover have respective thicknesses which satisfy the condition

-   -   cover thickness<intermediate layer thickness<envelope layer        thickness,        and have respective material hardnesses (Shore D) which satisfy        the condition    -   envelope layer material hardness<intermediate layer material        hardness>cover material hardness.        [2] The multi-piece solid golf ball of [1], wherein the envelope        layer is formed of a resin material which is a mixture        comprising:

100 parts by weight of a resin component composed of, in admixture,

-   -   a base resin of (a) an olefin-unsaturated carboxylic acid random        copolymer and/or a metal ion neutralization product of an        olefin-unsaturated carboxylic acid random copolymer mixed        with (b) an olefin-unsaturated carboxylic acid-unsaturated        carboxylic acid ester random terpolymer and/or a metal ion        neutralization product of an olefin-unsaturated carboxylic        acid-unsaturated carboxylic acid ester random terpolymer in a        weight ratio between 100:0 and 0:100, and    -   (e) a non-ionomeric thermoplastic elastomer in a weight ratio        between 100:0 and 50:50;

(c) 5 to 80 parts by weight of a fatty acid and/or fatty acid derivativehaving a molecular weight of 228 to 1500; and

(d) 0.1 to 17 parts by weight of a basic inorganic metal compoundcapable of neutralizing un-neutralized acid groups in the base resin andcomponent (c).

[3] The multi-piece solid golf ball of [1], wherein the overall core hasa surface and a center with a JIS-C hardness difference therebetween ofat least 23 but not more than 50, and wherein the overall core has adeflection (A) when compressed under a final load of 1,275 N (130 kgf)from an initial load of 98 N (10 kgf) and the inner core layer has adeflection (B) when compressed under a final load of 1,275 N (130 kgf)from an initial load of 98 N (10 kgf) such that the ratio (A)/(B) is atleast 0.50 but not more than 0.75.[4] The multi-piece solid golf ball of [1], wherein the inner core layerand/or the outer core layer contains an organosulfur compound.[5] The multi-piece solid golf ball of [1], wherein the inner core layerhas a diameter of at least 15 mm but not more than 28 mm.[6] The multi-piece solid golf ball of [1], wherein the rubber materialof the core includes a polybutadiene rubber synthesized with arare-earth catalyst or a Group VIII metal compound catalyst.[7] The multi-piece solid golf ball of [1], wherein the cover is formedby injection molding a single resin blend composed primarily of (A) athermoplastic polyurethane and (B) a polyisocyanate compound, whichresin blend contains a polyisocyanate compound in at least some portionof which all the isocyanate groups remain in an unreacted state.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a schematic sectional view showing a multi-piece solid golfball according to the invention.

FIG. 2 is a top view of a golf ball showing the arrangement of dimplesused in the examples of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described more fully below. The multi-piece solid golfball of the present invention, as shown in FIG. 1, is a golf ball Ghaving five or more layers, including an inner core layer 1 a, an outercore layer 1 b, an envelope layer 2 which encases the core, anintermediate layer 3 which encases the envelope layer, and a cover 4which encases the intermediate layer. The cover 4 typically has a largenumber of dimples D formed on the surface thereof. The core 1 and theintermediate layer 3 are not limited to single layers, and may each beformed of a plurality of two more layers.

In the golf ball of the invention, as shown in FIG. 1, the core isformed of two layers: an inner layer and an outer layer. The inner corelayer has a diameter of preferably at least 15 mm, more preferably atleast 16 mm, and even more preferably at least 17 mm, but preferably notmore than 28 mm, more preferably not more than 26 mm, and even morepreferably not more than 24 mm. An inner core layer diameter that is toosmall may result in too high a spin rate, possibly shortening thedistance traveled by the ball. On the other hand, if the diameter is toolarge, the outer core layer will have a correspondingly smallerthickness, which may result in a poor durability to repeated impact.Also, in the latter case, the compression (deflection) hardness of thecore is too low and the compression (deflection) hardness of the ball isalso too low, as a result of which the ball may have a smaller initialvelocity when played and thus fail to travel as far.

The outer core layer has a thickness of preferably at least 2 mm, morepreferably at least 3 mm, and even more preferably at least 5 mm, butpreferably not more than 12 mm, more preferably not more than 10 mm, andeven more preferably not more than 8 mm. If the outer core layer isthinner than the above range, the ball may have a poor durability torepeated impact and may fail to exhibit a spin rate-lowering effect onfull shots. On the other hand, if the outer core layer is too thick, thefeel on impact may become too hard and a spin rate-lowering effect maynot be achieved.

A material composed primarily of rubber may be used as the inner corelayer and outer core layer material having the above-described surfacehardness and deflection. The rubber material making up the outer corelayer surrounding the inner core layer may be of the same type or adifferent type as the material of the inner layer rubber. Specifically,the rubber composition may be prepared by using a base rubber as thechief component and blending therewith such ingredients as aco-crosslinking agent, an organic peroxide, an inert filler and anorganosulfur compound. It is preferable to use polybutadiene as the baserubber.

It is desirable for the polybutadiene serving as the rubber component tohave a cis-1,4-bond content on the polymer chain of at least 60 wt %,preferably at least 80 wt %, more preferably at least 90 wt %, and mostpreferably at least 95 wt %. Too low a cis-1,4-bond content among thebonds on the molecule may result in a lower resilience.

Also, the polybutadiene has a 1,2-vinyl bond content on the polymerchain of typically not more than 2%, preferably not more than 1.7%, andeven more preferably not more than 1.5%. Too high a 1,2-vinyl bondcontent may result in a lower resilience.

To obtain a molded and vulcanized rubber composition of good resilience,the polybutadiene used in the invention is preferably one synthesizedwith a rare-earth catalyst or a Group VIII metal compound catalyst.Polybutadiene synthesized with a rare-earth catalyst is especiallypreferred. The use, in particular, of a polybutadiene rubber synthesizedwith the above catalyst as the base rubber in the outer core layer issufficiently effective for the purposes of this invention. That is, whensuch a rubber is used, rubber having a high hardness can be obtained,facilitating the production of a core that is hard on the outside andsoft on the inside which is an object of the invention.

Such rare-earth catalysts are not subject to any particular limitation.Exemplary rare-earth catalysts include those made up of a combination ofa lanthanide series rare-earth compound with an organoaluminum compound,an alumoxane, a halogen-bearing compound and an optional Lewis base.

Examples of suitable lanthanide series rare-earth compounds includehalides, carboxylates, alcoholates, thioalcoholates and amides of atomicnumber 57 to 71 metals.

In the practice of the invention, the use of a neodymium catalyst inwhich a neodymium compound serves as the lanthanide series rare-earthcompound is particularly advantageous because it enables a polybutadienerubber having a high cis-1,4 bond content and a low 1,2-vinyl bondcontent to be obtained at an excellent polymerization activity. Suitableexamples of such rare-earth catalysts include those mentioned in JP-A11-35633, JP-A 11-164912 and JP-A 2002-293996.

To enhance the resilience, it is preferable for the polybutadienesynthesized using the lanthanide series rare-earth compound catalyst toaccount for at least 10 wt %, preferably at least 20 wt %, and morepreferably at least 40 wt %, of the rubber components.

Rubber components other than the above-described polybutadiene may beincluded in the base rubber insofar as the objects of the invention areattainable. Illustrative examples of rubber components other than theabove-described polybutadiene include other polybutadienes, and otherdiene rubbers, such as styrene-butadiene rubber, natural rubber,isoprene rubber and ethylene-propylene-diene rubber.

Examples of co-crosslinking agents include unsaturated carboxylic acidsand the metal salts of unsaturated carboxylic acids.

Specific examples of unsaturated carboxylic acids include acrylic acid,methacrylic acid, maleic acid and fumaric acid. Acrylic acid andmethacrylic acid are especially preferred.

The metal salts of unsaturated carboxylic acids, while not subject toany particular limitation, are exemplified by the above-mentionedunsaturated carboxylic acids neutralized with a desired metal ion.Specific examples include the zinc and magnesium salts of methacrylicacid and acrylic acid. The use of zinc acrylate is especially preferred.

The unsaturated carboxylic acid and/or metal salt thereof is included inan amount, per 100 parts by weight of the base rubber, of preferably atleast 10 parts by weight, more preferably at least 15 parts by weight,and even more preferably at least 20 parts by weight, but preferably notmore than 60 parts by weight, more preferably not more than 50 parts byweight, even more preferably not more than 45 parts by weight, and mostpreferably not more than 40 parts by weight. Too much may make the coretoo hard, giving the ball an unpleasant feel on impact, whereas toolittle may lower the rebound.

The organic peroxide may be a commercially available product, suitableexamples of which include Percumyl D (produced by NOF Corporation),Perhexa C-40 and Perhexa 3M (both produced by NOF Corporation), andLuperco 231XL (Atochem Co.). These may be used singly or as acombination of two or more thereof.

The amount of organic peroxide included per 100 parts by weight of thebase rubber is preferably at least 0.1 part by weight, more preferablyat least 0.3 part by weight, even more preferably at least 0.5 part byweight, and most preferably at least 0.7 part by weight, but preferablynot more than 5 parts by weight, more preferably not more than 4 partsby weight, even more preferably not more than 3 parts by weight, andmost preferably not more than 2 parts by weight. Too much or too littleorganic peroxide may make it impossible to achieve a ball having a goodfeel, durability and rebound.

Examples of suitable inert fillers include zinc oxide, barium sulfateand calcium carbonate. These may be used singly or as a combination oftwo or more thereof.

The amount of inert filler included per 100 parts by weight of the baserubber is preferably at least 1 part by weight, and more preferably atleast 5 parts by weight, but preferably not more than 50 parts byweight, more preferably not more than 40 parts by weight, and even morepreferably not more than 35 parts by weight. Too much or too littleinert filler may make it impossible to achieve a proper weight and agood rebound.

In addition, an antioxidant may be included if necessary. Illustrativeexamples of suitable commercial antioxidants include Nocrac NS-6, NocracNS-30 (both available from Ouchi Shinko Chemical Industry Co., Ltd.),and Yoshinox 425 (available from Yoshitomi Pharmaceutical Industries,Ltd.). These may be used singly or as a combination of two or morethereof.

The amount of antioxidant included per 100 parts by weight of the baserubber is preferably 0 or more part by weight, more preferably at least0.05 part by weight, and even more preferably at least 0.1 part byweight, but preferably not more than 3 parts by weight, more preferablynot more than 2 parts by weight, even more preferably not more than 1part by weight, and most preferably not more than 0.5 part by weight.Too much or too little antioxidant may make it impossible to achieve agood rebound and durability.

To confer a good rebound, it is preferable to include an organosulfurcompound within one or both of the inner core layer and the outer corelayer.

No particular limitation is imposed on the organosulfur compound,provided it improves the rebound of the golf ball. Exemplaryorganosulfur compounds include thiophenols, thionaphthols, halogenatedthiophenols, and metal salts thereof. Specific examples includepentachlorothiophenol, pentafluorothiophenol, pentabromothiophenol,p-chlorothiophenol, the zinc salt of pentachlorothiophenol, the zincsalt of pentafluorothiophenol, the zinc salt of pentabromothiophenol,the zinc salt of p-chlorothiophenol; and diphenylpolysulfides,dibenzylpolysulfides, dibenzoylpolysulfides, dibenzothiazoylpolysulfidesand dithiobenzoylpolysulfides having 2 to 4 sulfurs. The zinc salt ofpentachlorothiophenol is especially preferred.

It is recommended that the amount of the organosulfur compound includedper 100 parts by weight of the base rubber be preferably at least 0.05part by weight, more preferably at least 0.1 part by weight, and evenmore preferably at least 0.2 part by weight, but preferably not morethan 5 parts by weight, more preferably not more than 3 parts by weight,and even more preferably not more than 2.5 parts by weight. If too muchorganosulfur compound is included, further improvement in the rebound(especially on impact with a W#1) is unlikely to be achieved and thecore may become too soft, possibly resulting in a poor feel. On theother hand, if too little organosulfur compound is included, a reboundimproving effect is unlikely to be achieved.

The production of such a core made of two layers may entail molding theinner core layer by, for example, an ordinary method in which a sphereis formed under heating and compression at a temperature of at least140° C. but not more than 180° C. for a period of at least 10 minutesbut not more than 60 minutes. The method employed to form the outer corelayer on the surface of the inner core layer may involve forming a pairof half-cups from unvulcanized rubber sheet, placing and enclosing theinner core layer within the pair of half-cups, then molding under heatand pressure. For example, advantageous use can be made of a process inwhich initial vulcanization (semi-vulcanization) is carried out toproduce a pair of hemispherical cups, following which a prefabricatedinner core layer is placed in one of the hemispherical cups and coveredby the other hemispherical cup, and secondary vulcanization (completevulcanization) is subsequently carried out. Another preferred productionprocess involves forming the rubber composition while in an unvulcanizedstate into sheets so as to make a pair of outer core layer sheets, andshaping the sheets with a die having a hemispherical protrusion so as toproduce unvulcanized hemispherical cups. The pair of hemispherical cupsis then placed over a prefabricated inner core layer and formed into aspherical shape under heating and compression at a temperature of 140 to180*C for a period of 10 to 60 minutes.

In the invention, the diameter of the core (the overall core composed ofthe inner core layer and the outer core layer), while not subject to anyparticular limitation, is preferably at least 31 mm, more preferably atleast 32.5 mm, and even more preferably at least 34 mm, but preferablynot more than 38 mm, more preferably not more than 37 mm, and even morepreferably not more than 36 mm. A core diameter outside this range maylower the initial velocity of the ball or yield a less than adequatespin rate-lowering effect after the ball is hit, as a result of which anincreased distance may not be achieved.

The deflection when the core (the overall core composed of the innercore layer and outer core layer) is subjected to compressive loading,i.e., the deflection of the core when compressed under a final load of1,275 N (130 kgf) from an initial load of 98 N (10 kgf), while notsubject to any particular limitation, is preferably at least 3.0 mm,more preferably at least 3.3 mm, and even more preferably at least 3.5mm, but preferably not more than 7.0 mm, more preferably not more than6.0 mm, and even more preferably not more than 4.5 mm. If this value istoo high, the core may lack sufficient rebound, which may result in aless than satisfactory distance, the feel on impact may be too soft, andthe durability of the ball to cracking on repeated impact may worsen. Onthe other hand, if this value is too low, the ball may have anexcessively hard feel on full shots and the spin rate may be too high,as a result of which an increased distance may not be achieved.

In order to effectively achieve the objects of the invention, it isdesirable to optimize within a specific range the value obtained bydividing (A) the deflection of the overall core by (B) the deflection ofthe inner core layer. “Deflection of the inner core layer” refers hereinto, as with the deflection of the overall core, the deflection (mm) whencompressed under a final load of 1,275 N (130 kgf) from an initial loadof 98 N (10 kgf). The ratio (A)/(B) is preferably at least 0.50, morepreferably at least 0.53, and even more preferably at least 0.56, butpreferably not more than 0.75, more preferably not more than 0.70, andeven more preferably not more than 0.67. If this value is too small ortoo large, the spin rate of the ball when hit with a driver (W#1) mayrise and the initial velocity when the ball is actually played with aW#1 may decrease, as a result of which the desired distance may not beachieved.

The surface hardness of the core, while not subject to any particularlimitation, has a JIS-C hardness value of preferably at least 65, morepreferably at least 70, and even more preferably at least 75, butpreferably not more than 95, more preferably not more than 90, and evenmore preferably not more than 85. The center hardness of the core, whilenot subject to any particular limitation, has a JIS-C hardness value ofpreferably at least 20, more preferably at least 25, and even morepreferably at least 30, but preferably not more than 50, more preferablynot more than 40, and even more preferably not more than 35. If thecenter of the core is too hard, the ball may have an excessively highspin rate and thus may not travel as far as desired. Moreover, the ballmay have too hard a feel on impact. On the other hand, if the center ofthe core is too soft, the ball may have too low a rebound and thus maynot travel as far as desired. Moreover, the ball may have too soft afeel on impact, and may have a poor durability to cracking on repeatedimpact.

In the invention, it is critical that the outer core layer be formed soas to be harder than the inner core layer. Specifically, the hardnessdifference in JIS-C hardness units between the surface of the core andthe center of the core is preferably at least 23, more preferably atleast 25, and even more preferably at least 27, but preferably not morethan 50, more preferably not more than 45, and even more preferably notmore than 40. If the hardness difference is too small, the spinrate-lowering effect on shots with a W#1 may be insufficient and theball may thus not travel as far as desired. On the other hand, if thehardness difference is too large, the ball may have a smaller reboundand may thus not travel as far as desired, in addition to which thedurability to cracking on repeated impact may worsen.

Next, the envelope layer is described.

The material from which the envelope layer is formed has a hardness,expressed as the Durometer D hardness (measured with a type D durometerin accordance with ASTM D 2240), which, while not subject to anyparticular limitation, is preferably at least 40, more preferably atleast 47, and even more preferably at least 50, but preferably not morethan 62, more preferably not more than 60, and even more preferably notmore than 58. If the envelope layer material is softer than the aboverange, the ball may have too much spin receptivity on full shots, as aresult of which an increased distance may not be achieved. On the otherhand, if this material is harder than the above range, the durability ofthe ball to cracking under repeated impact may worsen and the ball mayhave too hard a feel when played. The envelope layer has a thicknesswhich, while not subject to any particular limitation, is preferably atleast 1.0 mm, more preferably at least 1.2 mm, and even more preferablyat least 1.4 mm, but preferably not more than 4.0 mm, more preferablynot more than 3.0 mm, and even more preferably not more than 2.0 mm.Outside this range, the spin rate-lowering effect on shots with a driver(W#1) may be inadequate, as a result of which an increased distance maynot be achieved.

The envelope layer has a surface hardness, expressed as the JIS-Chardness, which, while not subject to any particular limitation, ispreferably at least 75, more preferably at least 79, and even morepreferably at least 83, but preferably not more than 98, more preferablynot more than 95, and even more preferably not more than 90. At asurface hardness lower than this range, the ball may have too much spinreceptivity on full shots, as a result of which an increased distancemay not be achieved. On the other hand, if the surface hardness ishigher than the above range, the durability of the ball to crackingunder repeated impact may worsen and the ball may have too hard a feelwhen played. It is desirable for the surface of the envelope layer to besofter than the surface of the intermediate layer. While no particularlimitation is imposed on the degree to which it is softer, thedifference in JIS-C hardness units is preferably at least 3, morepreferably at least 5, and even more preferably at least 7, butpreferably not more than 20, more preferably not more than 18, and evenmore preferably not more than 16. Outside this range, if the surface ofthe envelope layer is too much softer than the surface of theintermediate layer, the rebound of the ball may decrease or the spinrate may become excessive, as a result of which an increased distancemay not be achieved.

Moreover, it is desirable that the surface of the envelope layer not bemade softer than the surface of the core. While no particular limitationis imposed on the degree thereof, the value represented by (JIS-Chardness of envelope layer surface—JIS-C hardness of core surface) inJIS-C hardness units is preferably at least 0, and more preferably atleast 1, but preferably not more than 20, more preferably not more than15, and even more preferably not more than 10. If the surface of theenvelope layer is instead softer than the core surface, the spinrate-lowering effect on shots with a driver may be inadequate, as aresult of which an increased distance may not be achieved. On the otherhand, if the surface of the envelope layer is harder than the coresurface to a degree that falls outside the above range, the feel of theball on full shots may be too hard and the durability of the ball tocracking on repeated impact may worsen.

The envelope layer in the invention is formed primarily of a resinmaterial. The resin material in the envelope layer, while not subject toany particular limitation, preferably includes as an essential componenta base resin composed of, in admixture, specific amounts of (a) anolefin-unsaturated carboxylic acid random copolymer and/or a metal ionneutralization product of an olefin-unsaturated carboxylic acid randomcopolymer and (b) an olefin-unsaturated carboxylic acid-unsaturatedcarboxylic acid ester random terpolymer and/or a metal ionneutralization product of an olefin-unsaturated carboxylicacid-unsaturated carboxylic acid ester random terpolymer. That is, inthe present invention, by using the material described below as thepreferred material in the envelope layer, the spin rate on shots with aW#1 can be lowered, enabling a longer distance to be achieved.

The olefin in the above base resin, whether in component (a) orcomponent (b), has a number of carbons which is preferably at least 2but preferably not more than 8, and more preferably not more than 6.Specific examples include ethylene, propylene, butene, pentene, hexene,heptene and octene. Ethylene is especially preferred.

Examples of unsaturated carboxylic acids include acrylic acid,methacrylic acid, maleic acid and fumaric acid. Acrylic acid andmethacrylic acid are especially preferred.

Moreover, the unsaturated carboxylic acid ester is preferably a loweralkyl ester of the above unsaturated carboxylic acid. Specific examplesinclude methyl methacrylate, ethyl methacrylate, propyl methacrylate,butyl methacrylate, methyl acrylate, ethyl acrylate, propyl acrylate andbutyl acrylate. Butyl acrylate (n-butyl acrylate, i-butyl acrylate) isespecially preferred.

The olefin-unsaturated carboxylic acid random copolymer of component (a)and the olefin-unsaturated carboxylic acid-unsaturated carboxylic acidester random terpolymer of component (b) (the copolymers in components(a) and (b) are referred to collectively below as “random copolymers”)can each be obtained by preparing the above-mentioned materials andcarrying out random copolymerization by a known method.

It is recommended that the above random copolymers have unsaturatedcarboxylic acid contents (acid contents) that are controlled. Here, itis recommended that the content of unsaturated carboxylic acid presentin the random copolymer serving as component (a) be preferably at least4 wt %, more preferably at least 6 wt %, even more preferably at least 8wt %, and most preferably at least 10 wt %, but preferably not more than30 wt %, more preferably not more than 20 wt %, even more preferably notmore than 18 wt %, and most preferably not more than 15 wt %.

Similarly, it is recommended that the content of unsaturated carboxylicacid present in the random copolymer serving as component (b) bepreferably at least 4 wt %, more preferably at least 6 wt %, and evenmore preferably at least 8 wt %, but preferably not more than 15 wt %,more preferably not more than 12 wt %, and even more preferably not morethan 10 wt %. If the acid content of the random copolymer is too low,the resilience may decrease, whereas if it is too high, theprocessability of the envelope layer-forming resin material maydecrease.

The metal ion neutralization product of the olefin-unsaturatedcarboxylic acid random copolymer of component (a) and the metal ionneutralization product of the olefin-unsaturated carboxylicacid-unsaturated carboxylic acid ester random terpolymer of component(b) (the metal ion neutralization products of the copolymers incomponents (a) and (b) are referred to collectively below as “metal ionneutralization products of the random copolymers”) can be obtained byneutralizing some of the acid groups on the random copolymers with metalions.

Illustrative examples of metal ions for neutralizing the acid groupsinclude Na⁺, K⁺, Li⁺, Zn⁺⁺, Cu⁺⁺, Mg⁺⁺, Ca⁺⁺, Co⁺⁺, Ni⁺⁺ and Pb⁺⁺. Ofthese, preferred use can be made of, for example, Na⁺, Li⁺, Zn⁺⁺ andMg⁺⁺. To improve resilience, the use of Na⁺ is even more preferred.

The above metal ion neutralization products of the random copolymers maybe obtained by neutralizing the random copolymers with the foregoingmetal ions. For example, use may be made of a method in whichneutralization is carried out with a compound such as a formate,acetate, nitrate, carbonate, bicarbonate, oxide, hydroxide or alkoxideof the above-mentioned metal ions. No particular limitation is imposedon the degree of neutralization of the random copolymer by these metalions.

Sodium ion-neutralized ionomer resins may be suitably used as the abovemetal ion neutralization products of the random copolymers to increasethe melt flow rate of the material. In this way, adjustment of thematerial to the subsequently described optimal melt flow rate is easy,enabling the moldability to be improved.

Commercially available products may be used as the base resins of abovecomponents (a) and (b). Illustrative examples of the random copolymer incomponent (a) include Nucrel 1560, Nucrel 1214 and Nucrel 1035 (allproducts of DuPont-Mitsui Polychemicals Co., Ltd.), and Escor 5200,Escor 5100 and Escor 5000 (all products of ExxonMobil Chemical).Illustrative examples of the random copolymer in component (b) includeNucrel AN4311 and Nucrel AN4318 (both products of DuPont-MitsuiPolychemicals Co., Ltd.), and Escor ATX325, Escor ATX320 and EscorATX310 (all products of ExxonMobil Chemical).

Illustrative examples of the metal ion neutralization product of therandom copolymer in component (a) include Himilan 1554, Himilan 1557,Himilan 1601, Himilan 1605, Himilan 1706 and Himilan AM7311 (allproducts of DuPont-Mitsui Polychemicals Co., Ltd.), Surlyn 7930 (E.I.DuPont de Nemours & Co.), and Iotek 3110 and Iotek 4200 (both productsof ExxonMobil Chemical). Illustrative examples of the metal ionneutralization product of the random copolymer in component (b) includeHimilan 1855, Himilan 1856 and Himilan AM7316 (all products ofDuPont-Mitsui Polychemicals Co., Ltd.), Surlyn 6320, Surlyn 8320, Surlyn9320 and Surlyn 8120 (all products of E.I. DuPont de Nemours & Co.), andIotek 7510 and Iotek 7520 (both products of ExxonMobil Chemical).Sodium-neutralized ionomer resins that are suitable as the metal ionneutralization product of the random copolymer include Himilan 1605,Himilan 1601 and Himilan 1555.

When preparing the above-described base resin, component (a) andcomponent (b) are admixed in a weight ratio of between 100:0 and 0:100,preferably between 100:0 and 25:75, more preferably between 100:0 and50:50, even more preferably between 100:0 and 75:25, and most preferably100:0. If too little component (a) is included, the molded materialobtained therefrom may have a decreased resilience.

In addition, the processability of the base resin can be furtherimproved by also adjusting the ratio in which the random copolymers andthe metal ion neutralization products of the random copolymers areadmixed when preparing the base resin as described above. It isrecommended that the weight ratio of the random copolymers to the metalion neutralization products of the random copolymers be between 0:100and 60:40, preferably between 0:100 and 40:60, more preferably between0:100 and 20:80, and even more preferably 0:100. The addition of toomuch random copolymer may lower the processability during mixing.

Component (e) described below may be added to the base resin. Component(e) is a non-ionomeric thermoplastic elastomer. The purpose of thiscomponent is to further improve the feel of the ball on impact and therebound. Examples include olefin elastomers, styrene elastomers,polyester elastomers, urethane elastomers and polyamide elastomers. Tofurther increase the rebound, it is preferable to use a polyesterelastomer or an olefin elastomer. The use of an olefin elastomercomposed of a thermoplastic block copolymer which includes crystallinepolyethylene blocks as the hard segments is especially preferred.

A commercially available product may be used as component (e).Illustrative examples include Dynaron (JSR Corporation) and thepolyester elastomer Hytrel (DuPont-Toray Co., Ltd.).

It is recommended that component (e) be included in an amount, per 100parts by weight of the base resin of the invention, of preferably atleast 0 part by weight, more preferably at least 5 parts by weight, evenmore preferably at least 10 parts by weight, and most preferably atleast 20 parts by weight, but preferably not more than 100 parts byweight, more preferably not more than 60 parts by weight, even morepreferably not more than 50 parts by weight, and most preferably notmore than 40 parts by weight. Too much component (e) will lower thecompatibility of the mixture, possibly resulting in a substantialdecline in the durability of the golf ball.

Next, component (c) described below may be added to the base resin.Component (c) is a fatty acid or fatty acid derivative having amolecular weight of at least 228 but not more than 1500. Compared withthe base resin, this component has a very low molecular weight and, bysuitably adjusting the melt viscosity of the mixture, helps inparticular to improve the flow properties. Component (c) includes arelatively high content of acid groups (or derivatives thereof), and iscapable of suppressing an excessive loss in resilience.

The fatty acid or fatty acid derivative of component (c) has a molecularweight of at least 228, preferably at least 256, more preferably atleast 280, and even more preferably at least 300, but not more than1500, preferably not more than 1000, even more preferably not more than600, and most preferably not more than 500. If the molecular weight istoo low, the heat resistance cannot be improved. On the other hand, ifthe molecular weight is too high, the flow properties cannot beimproved.

The fatty acid or fatty acid derivative of component (c) may be anunsaturated fatty acid (or derivative thereof) containing a double bondor triple bond on the alkyl moiety, or it may be a saturated fatty acid(or derivative thereof) in which the bonds on the alkyl moiety are allsingle bonds. It is recommended that the number of carbons on themolecule be preferably at least 18, more preferably at least 20, evenmore preferably at least 22, and most preferably at least 24, butpreferably not more than 80, more preferably not more than 60, even morepreferably not more than 40, and most preferably not more than 30. Toofew carbons may make it impossible to improve the heat resistance andmay also make the acid group content so high as to diminish theflow-improving effect due to interactions with acid groups present inthe base resin. On the other hand, too many carbons increases themolecular weight, which may keep a distinct flow-improving effect fromappearing.

Specific examples of the fatty acid of component (c) include myristicacid, palmitic acid, stearic acid, 12-hydroxystearic acid, behenic acid,oleic acid, linoleic acid, linolenic acid, arachidic acid and lignocericacid. Of these, stearic acid, arachidic acid, behenic acid andlignoceric acid are preferred. Behenic acid is especially preferred.

The fatty acid derivative of component (c) is exemplified by metallicsoaps in which the proton on the acid group of the fatty acid has beenreplaced with a metal ion. Examples of the metal ion include Na⁺, Li⁺,Ca⁺⁺, Mg⁺⁺, Zn⁺⁺, Mn⁺⁺, Al⁺⁺⁺, Ni⁺⁺, Fe⁺⁺, Fe⁺⁺⁺, Cu⁺⁺, Sn⁺⁺, Pb⁺⁺ andCo⁺⁺. Of these, Ca⁺⁺, Mg⁺⁺ and Zn⁺⁺ are especially preferred.

Specific examples of fatty acid derivatives that may be used ascomponent (c) include magnesium stearate, calcium stearate, zincstearate, magnesium 12-hydroxystearate, calcium 12-hydroxystearate, zinc12-hydroxystearate, magnesium arachidate, calcium arachidate, zincarachidate, magnesium behenate, calcium behenate, zinc behenate,magnesium lignocerate, calcium lignocerate and zinc lignocerate. Ofthese, magnesium stearate, calcium stearate, zinc stearate, magnesiumarachidate, calcium arachidate, zinc arachidate, magnesium behenate,calcium behenate, zinc behenate, magnesium lignocerate, calciumlignocerate and zinc lignocerate are preferred.

Component (d) may be added as a basic inorganic metal compound capableof neutralizing acid groups in the base resin and in component (c). Ifcomponent (d) is not included, when a metal soap-modified ionomer resin(e.g., the metal soap-modified ionomer resins cited in theabove-mentioned patent publications) is used alone, the metallic soapand un-neutralized acid groups present on the ionomer resin undergoexchange reactions during mixture under heating, generating a largeamount of fatty acid. Because the fatty acid has a low thermal stabilityand readily vaporizes during molding, it may cause molding defects.Moreover, if the fatty acid thus generated deposits on the surface ofthe molded material, it may substantially lower paint film adhesion andmay have other undesirable effects such as lowering the resilience ofthe resulting molded material.

Accordingly, to solve this problem, the envelope layer-forming resinmaterial includes also, as an essential component, a basic inorganicmetal compound (d) which neutralizes the acid groups present in the baseresin and component (c), in this way improving the resilience of themolded material.

That is, by including component (d) as an essential ingredient in thematerial, not only are the acid groups in the base resin and component(c) neutralized, through synergistic effects from the optimal additionof each of these components it is possible as well to increase thethermal stability of the mixture and give it a good moldability, andalso to enhance the resilience.

Here, it is recommended that the basic inorganic metal compound used ascomponent (d) be a compound which has a high reactivity with the baseresin and contains no organic acids in the reaction by-products, thusenabling the degree of neutralization of the mixture to be increasedwithout a loss of thermal stability.

Illustrative examples of the metal ion in the basic inorganic metalcompound serving as component (d) include Li⁺, Na⁺, K⁺, Ca⁺⁺, Mg⁺⁺,Zn⁺⁺, Al⁺⁺⁺, Ni⁺⁺, Fe⁺⁺, Fe⁺⁺⁺, Cu⁺⁺, Mn⁺⁺, Sn⁺⁺, Pb⁺⁺ and Co⁺⁺. Knownbasic inorganic fillers containing these metal ions may be used as thebasic inorganic metal compound. Specific examples include magnesiumoxide, magnesium hydroxide, magnesium carbonate, zinc oxide, sodiumhydroxide, sodium carbonate, calcium oxide, calcium hydroxide, lithiumhydroxide and lithium carbonate. In particular, a hydroxide or amonoxide is recommended. Calcium hydroxide and magnesium oxide, whichhave a high reactivity with the base resin, are more preferred. Calciumhydroxide is especially preferred.

Because the above-described resin material is arrived at by blendingspecific respective amounts of components (c) and (d) with the resincomponent, i.e., the base resin containing specific respective amountsof components (a) and (b) in combination with optional component (e),this material has excellent thermal stability, flow properties andmoldability, and can impart the molded material with a markedly improvedresilience.

Components (c) and (d) are included in respective amounts, per 100 partsby weight of the resin component suitably formulated from components(a), (b) and (e), of at least 5 parts by weight, preferably at least 10parts by weight, more preferably at least 15 parts by weight, and evenmore preferably at least 18 parts by weight, but not more than 80 partsby weight, preferably not more than 40 parts by weight, more preferablynot more than 25 parts by weight, and even more preferably not more than22 parts by weight, of component (c); and at least 0.1 part by weight,preferably at least 0.5 part by weight, more preferably at least 1 partby weight, and even more preferably at least 2 parts by weight, but notmore than 17 parts by weight, preferably not more than 15 parts byweight, more preferably not more than 13 parts by weight, and even morepreferably not more than 10 parts by weight, of component (d). Toolittle component (c) lowers the melt viscosity, resulting in inferiorprocessability, whereas too much lowers the durability. Too littlecomponent (d) fails to improve thermal stability and resilience, whereastoo much instead lowers the heat resistance of the golf ball-formingmaterial due to the presence of excess basic inorganic metal compound.

In the above-described resin material formulated from the respectiveabove-indicated amounts of the resin component and components (c) and(d), it is recommended that at least 50 mol %, preferably at least 60mol %, more preferably at least 70 mol %, and even more preferably atleast 80 mol %, of the acid groups be neutralized. Such a high degree ofneutralization makes it possible to more reliably suppress the exchangereactions that cause trouble when only a base resin and a fatty acid orfatty acid derivative are used as in the above-cited prior art, thuspreventing the generation of fatty acid. As a result, there is obtaineda resin material of substantially improved thermal stability and goodprocessability which can provide molded products of much betterresilience than prior-art ionomer resins.

“Degree of neutralization,” as used above, refers to the degree ofneutralization of acid groups present within the mixture of the baseresin and the fatty acid or fatty acid derivative serving as component(c), and differs from the degree of neutralization of the ionomer resinitself when an ionomer resin is used as the metal ion neutralizationproduct of a random copolymer in the base resin. A mixture according tothe invention having a certain degree of neutralization, when comparedwith an ionomer resin alone having the same degree of neutralization,contains a very large number of metal ions. This large number of metalions increases the density of ionic crosslinks which contribute toimproved resilience, making it possible to confer the molded productwith excellent resilience.

To more reliably achieve a material having both a high degree ofneutralization and good flow properties, it is recommended that the acidgroups in the above-described mixture be neutralized with transitionmetal ions and with alkali metal and/or alkaline earth metal ions.Although neutralization with transition metal ions results in a weakerionic cohesion than neutralization with alkali metal and alkaline earthmetal ions, by using these different types of ions together toneutralize acid groups in the mixture, a substantial improvement can bemade in the flow properties.

It is recommended that the molar ratio between the transition metal ionsand the alkali metal and/or alkaline earth metal ions be in a range oftypically 10:90 to 90:10, preferably 20:80 to 80:20, more preferably30:70 to 70:30, and even more preferably 40:60 to 60:40. Too low a molarratio of transition metal ions may fail to provide a sufficientflow-improving effect. On the other hand, a transition metal ion molarratio which is too high may lower the resilience.

Examples of the metal ions include, but are not limited to, zinc ions asthe transition metal ions and at least one type of ion selected fromamong sodium, lithium and magnesium ions as the alkali metal or alkalineearth metal ions.

A known method may be used to obtain a mixture in which the desiredamount of acid groups have been neutralized with transition metal ionsand alkali metal or alkaline earth metal ions. Specific examples ofmethods of neutralization with transition metal ions, particularly zincions, include a method which uses a zinc soap as the fatty acidderivative, a method which uses a zinc ion neutralization product (e.g.,a zinc ion-neutralized ionomer resin) when formulating components (a)and (b) as the base resin, and a method which uses a zinc compound suchas zinc oxide as the basic inorganic metal compound of component (d).

The resin material should preferably have a melt flow rate adjusted toensure flow properties that are particularly suitable for injectionmolding, and thus improve moldability. Specifically, it is recommendedthat the melt flow rate (MFR), as measured according to JIS-K7210 at atemperature of 190° C. and under a load of 21.18 N (2.16 kgf), be set topreferably at least 0.6 dg/min, more preferably at least 0.7 dg/min,even more preferably at least 0.8 dg/min, and most preferably at least 2dg/min, but preferably not more than 20 dg/min, more preferably not morethan 10 dg/min, even more preferably not more than 5 dg/min, and mostpreferably not more than 3 dg/min. Too high or low a melt flow rate mayresult in a substantial decline in processability.

Illustrative examples of the envelope layer material include thosehaving the trade names HPF 1000, HPF 2000 and HPF AD1027, as well as theexperimental material HPF SEP1264-3, all produced by E.I. DuPont deNemours & Co.

Next, the intermediate layer is described.

The material from which the intermediate layer is formed has a hardness,expressed as the Durometer D hardness (measured with a type D durometerin accordance with ASTM D 2240), which, while not subject to anyparticular limitation, is preferably at least 50, more preferably atleast 55, and even more preferably at least 60, but preferably not morethan 70, more preferably not more than 66, and even more preferably notmore than 63. If the intermediate layer material is softer than theabove range, the ball may have too much spin receptivity on full shots,as a result of which an increased distance may not be attained. On theother hand, if this material is harder than the above range, thedurability of the ball to cracking on repeated impact may worsen and theball may have too hard a feel when played with a putter or on shortapproach shots. The intermediate layer has a thickness which, while notsubject to any particular limitation, is preferably at least 0.7 mm,more preferably at least 0.9 mm, and even more preferably at least 1.1mm, but preferably not more than 2.0 mm, more preferably not more than1.7 mm, and even more preferably not more than 1.4 mm. Outside thisrange, the spin rate-lowering effect on shots with a driver (W#1) may beinadequate, as a result of which an increased distance may not beachieved. Moreover, a thickness lower than the above range may worsenthe durability to cracking on repeated impact.

The intermediate layer is formed primarily of a resin material which maybe the same as or different from the above-described material used toform the envelope layer. An ionomer resin is especially preferred.Specific examples include sodium-neutralized ionomer resins availableunder the trade name designations Himilan 1605, Himilan 1601 and Surlyn8120, and zinc-neutralized ionomer resins such as Himilan 1557 andHimilan 1706. These may be used singly or as a combination of two ormore thereof.

An embodiment in which the intermediate layer material is composedprimarily of, in admixture, both a zinc-neutralized ionomer resin and asodium-neutralized ionomer resin is especially preferable for attainingthe objects of the invention. The mixing ratio, expressed aszinc-neutralized resin/sodium-neutralized resin (weight ratio), isgenerally from 25/75 to 75/25, preferably from 35/65 to 65/35, and morepreferably from 45/55 to 55/45.

Outside this range, the ball rebound may be too low, as a result ofwhich the desired distance may not be achieved, the durability torepeated impact at normal temperature may worsen, and the durability tocracking at low temperatures (below 0° C.) may worsen.

The surface of the intermediate layer, i.e., the surface of the spherecomposed of the core enclosed by the envelope layer and the intermediatelayer, has a JIS-C hardness which, while not subject to any particularlimitation, is preferably at least 85, more preferably at least 90, andeven more preferably at least 95, but preferably not more than 100, morepreferably not more than 99, and even more preferably not more than 98.If the surface of the intermediate layer is softer than the above range,the ball may have too much spin receptivity on full shots, as a resultof which an increased distance may not be achieved. On the other hand,if it is harder than the above range, the durability of the ball tocracking on repeated impact may worsen and the ball may have too hard afeel when played with a putter or on short approach shots.

The intermediate layer is typically formed so as to have a surfacehardness which is higher than the surface hardness of the core.Specifically, the intermediate layer is formed so as to have a surfacehardness which is preferably at least 1, more preferably at least 5, andeven more preferably at least 9, but preferably not more than 30, morepreferably not more than 20, and even more preferably not more than 16JIS-C units higher than the JIS-C hardness at the surface of theenvelope layer.

Also, as described in more detail below, the intermediate layer istypically formed so as to have a higher surface hardness than the cover.

To increase adhesion between the intermediate layer material and thepolyurethane used in the subsequently described cover, it is desirableto abrade the surface of the intermediate layer. In addition, it ispreferable to apply a primer (adhesive) to the surface of theintermediate layer following such abrasion or to add an adhesionreinforcing agent to the intermediate layer material. Examples ofadhesion reinforcing agents that may be incorporated in the materialinclude organic compounds such as 1,3-butanediol and trimethylolpropane,and oligomers such as polyethylene glycol and polyhydroxy polyolefinoligomers. The use of trimethylolpropane or a polyhydroxy polyolefinoligomer is especially preferred. Examples of commercially availableproducts include trimethylolpropane produced by Mitsubishi Gas ChemicalCo., Ltd. and polyhydroxy polyolefin oligomers produced by MitsubishiChemical Corporation (under the trade name designation Polytail H;number of main-chain carbons, 150 to 200; with hydroxyl groups at theends).

Next, the cover is described. As used herein, the term “cover” denotesthe outermost layer of the ball construction, and excludes what arereferred to herein as the intermediate layer and the envelope layer.

The cover material has a hardness, expressed as the Durometer Dhardness, which, while not subject to any particular limitation, ispreferably at least 40, more preferably at least 43, and even morepreferably at least 46, but preferably not more than 60, more preferablynot more than 57, and even more preferably not more than 54. At ahardness below this range, the ball tends to take on too much spin onfull shots, as a result of which an increased distance may not beachieved. On the other hand, at a hardness above this range, on approachshots, the ball lacks spin receptivity and thus may have an inadequatecontrollability even when played by a professional or other skilledgolfer.

The thickness of the cover, while not subject to any particularlimitation, is preferably at least 0.3 mm, more preferably at least 0.5mm, and even more preferably at least 0.7 mm, but preferably not morethan 1.5 mm, more preferably not more than 1.2 mm, and even morepreferably not more than 1.0 mm. If the cover is thicker than the aboverange, the ball may have an inadequate rebound on shots with a driver(W#1) or the spin rate may be too high, as a result of which anincreased distance may not be achieved. Conversely, if the cover isthinner than the above range, the ball may have a poor scuff resistanceand inadequate controllability even when played by a professional orother skilled golfer.

In the practice of invention, the cover material is not subject to anyparticular limitation, although it is preferable for the cover to beformed primarily of a thermoplastic resin or a thermoplastic elastomer.The use of a polyurethane as the primary material is especiallypreferred because it enables the intended effects of the invention,i.e., both a good controllability and a good scuff resistance, to beachieved.

The polyurethane used as the cover material, while not subject to anyparticular limitation, is preferably a thermoplastic polyurethane,particularly from the standpoint of amenability to mass production.

It is preferable to use a specific thermoplastic polyurethanecomposition composed primarily of (A) a thermoplastic polyurethane and(B) a polyisocyanate compound. This resin blend is described below.

To fully exhibit the advantageous effects of the invention, a necessaryand sufficient amount of unreacted isocyanate groups should be presentin the cover resin material. Specifically, it is recommended that thetotal weight of above components A and B combined be at least 60%, andpreferably at least 70%, of the overall weight of the cover. ComponentsA and B are described in detail below.

The thermoplastic polyurethane serving as component A has a structurewhich includes soft segments made of a polymeric polyol that is along-chain polyol (polymeric glycol), and hard segments made of a chainextender and a polyisocyanate compound. Here, the long-chain polyol usedas a starting material is not subject to any particular limitation, andmay be any that is used in the prior art relating to thermoplasticpolyurethanes. Exemplary long-chain polyols include polyester polyols,polyether polyols, polycarbonate polyols, polyester polycarbonatepolyols, polyolefin polyols, conjugated diene polymer-based polyols,castor oil-based polyols, silicone-based polyols and vinyl polymer-basedpolyols. These long-chain polyols may be used singly or as combinationsof two or more thereof. Of the long-chain polyols mentioned here,polyether polyols are preferred because they enable the synthesis ofthermoplastic polyurethanes having a high rebound resilience andexcellent low-temperature properties.

Illustrative examples of the above polyether polyol includepoly(ethylene glycol), poly(propylene glycol), poly(tetramethyleneglycol) and poly(methyltetramethylene glycol) obtained by thering-opening polymerization of cyclic ethers. The polyether polyol maybe used singly or as a combination of two or more thereof. Of the above,poly(tetramethylene glycol) and/or poly(methyltetramethylene glycol) arepreferred.

It is preferable for these long-chain polyols to have a number-averagemolecular weight in a range of 1,500 to 5,000. By using a long-chainpolyol having a number-average molecular weight within this range, golfballs made with a thermoplastic polyurethane composition havingexcellent properties such as resilience and manufacturability can bereliably obtained. The number-average molecular weight of the long-chainpolyol is more preferably in a range of 1,700 to 4,000, and even morepreferably in a range of 1,900 to 3,000.

As used herein, “number-average molecular weight of the long-chainpolyol” refers to the number-average molecular weight computed based onthe hydroxyl number measured in accordance with JIS K-1557.

Suitable chain extenders include those used in the prior art relating tothermoplastic polyurethanes. For example, low-molecular-weight compoundswhich have a molecular weight of 400 or less and bear on the moleculetwo or more active hydrogen atoms capable of reacting with isocyanategroups are preferred. Illustrative, non-limiting, examples of the chainextender include 1,4-butylene glycol, 1,2-ethylene glycol,1,3-butanediol, 1,6-hexanediol and 2,2-dimethyl-1,3-propanediol. Ofthese chain extenders, aliphatic diols having 2 to 12 carbons arepreferred, and 1,4-butylene glycol is especially preferred.

The polyisocyanate compound is not subject to any particular limitation;preferred use may be made of one that is used in the prior art relatingto thermoplastic polyurethanes. Specific examples include one or moreselected from the group consisting of 4,4′-diphenylmethane diisocyanate,2,4-toluene diisocyanate, 2,6-toluene diisocyanate, p-phenylenediisocyanate, xylylene diisocyanate, naphthylene-1,5-diisocyanate,tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate,dicyclohexylmethane diisocyanate, tetramethylene diisocyanate,hexamethylene diisocyanate, isophorone diisocyanate, norbornenediisocyanate, trimethylhexamethylene diisocyanate and dimer aciddiisocyanate. Depending on the type of isocyanate used, the crosslinkingreaction during injection molding may be difficult to control. In thepractice of the invention, to provide a balance between stability at thetime of production and the properties that are manifested, it is mostpreferable to use 4,4′-diphenylmethane diisocyanate, which is anaromatic diisocyanate.

It is most preferable for the thermoplastic polyurethane serving asabove component A to be a thermoplastic polyurethane synthesized using apolyether polyol as the long-chain polyol, using an aliphatic diol asthe chain extender, and using an aromatic diisocyanate as thepolyisocyanate compound. It is desirable, though not essential, for thepolyether polyol to be a polytetramethylene glycol having anumber-average molecular weight of at least 1,900, for the chainextender to be 1,4-butylene glycol, and for the aromatic diisocyanate tobe 4,4′-diphenylmethane diisocyanate.

The mixing ratio of active hydrogen atoms to isocyanate groups in theabove polyurethane-forming reaction can be controlled within a desirablerange so as to make it possible to obtain a golf ball which is composedof a thermoplastic polyurethane composition and has various improvedproperties, such as rebound, spin performance, scuff resistance andmanufacturability. Specifically, in preparing a thermoplasticpolyurethane by reacting the above long-chain polyol, polyisocyanatecompound and chain extender, it is desirable to use the respectivecomponents in proportions such that the amount of isocyanate groups onthe polyisocyanate compound per mole of active hydrogen atoms on thelong-chain polyol and the chain extender is from 0.95 to 1.05 moles.

No particular limitation is imposed on the method of preparing thethermoplastic polyurethane used as component A. Production may becarried out by either a prepolymer process or a one-shot process inwhich the long-chain polyol, chain extender and polyisocyanate compoundare used and a known urethane-forming reaction is effected. Of these, aprocess in which melt polymerization is carried out in a substantiallysolvent-free state is preferred. Production by continuous meltpolymerization using a multiple screw extruder is especially preferred.

Illustrative examples of the thermoplastic polyurethane that may be usedas component A include commercial products such as Pandex T8295, PandexT8290 and Pandex T8260 (all available from DIC Bayer Polymer, Ltd.).

Next, concerning the polyisocyanate compound used as component B, it isessential that, in at least some portion thereof, all the isocyanategroups on the molecule remain in an unreacted state. That is,polyisocyanate compound in which all the isocyanate groups on themolecule remain in a completely free state should be present, and such apolyisocyanate compound may be present together with polyisocyanatecompound in which only one end of the molecule is in a free state.

Various types of isocyanates may be employed without particularlimitation as the polyisocyanate compound. Illustrative examples includeone or more selected from the group consisting of 4,4′-diphenylmethanediisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate,p-phenylene diisocyanate, xylylene diisocyanate,naphthylene-1,5-diisocyanate, tetramethylxylene diisocyanate,hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate,tetramethylene diisocyanate, hexamethylene diisocyanate, isophoronediisocyanate, norbornene diisocyanate, trimethylhexamethylenediisocyanate and dimer acid diisocyanate. Of the above group ofisocyanates, the use of 4,4′-diphenylmethane diisocyanate,dicyclohexylmethane diisocyanate and isophorone diisocyanate ispreferable in terms of the balance between the influence onprocessability of such effects as the rise in viscosity that accompaniesthe reaction with the thermoplastic polyurethane serving as component Aand the physical properties of the resulting golf ball cover material.

In the practice of the invention, although not an essential constituent,a thermoplastic elastomer other than the above-described thermoplasticpolyurethane may be included as component C together with components Aand B. Including this component C in the above resin composition enablesthe fluidity of the resin composition to be further improved and enablesimprovements to be made in various properties required of golf ballcover materials, such as resilience and scuff resistance.

In addition to the above resin components, various optional additivesmay be included in the above-described resin materials for the envelopelayer, the intermediate layer and the cover. Such additives include, forexample, pigments, dispersants, antioxidants, ultraviolet absorbers,ultraviolet stabilizers, parting agents, plasticizers, and inorganicfillers (e.g., zinc oxide, barium sulfate, titanium dioxide).

Thickness Relationship Between Envelope Layer, Intermediate Layer andCover

In the present invention, it is critical for the thicknesses of theenvelope layer, the intermediate layer and the cover to satisfy thecondition

-   -   cover thickness<intermediate layer thickness<envelope layer        thickness.

By having the core diameter be at least 31 mm and also suitablyselecting the relative thicknesses of these respective layers, there canbe obtained a golf ball which exhibits good flight performance,controllability, durability and feel. Should the cover be thicker thanthe intermediate layer, the ball rebound will decrease or the ball willhave excessive spin receptivity on full shots, as a result of which anincreased distance will not be attainable. Should the envelope layer bethinner than the intermediate layer, the spin rate-lowering effect willbe inadequate, preventing the desired distance from being achieved.

Relationship Between Material Hardnesses of Envelope Layer, IntermediateLayer and Cover

In the present invention, it is critical for the material hardnesses(Shore D) of the envelope layer, the intermediate layer and the cover tosatisfy the condition:

-   -   envelope layer material hardness<intermediate layer material        hardness>cover material hardness.

The multi-piece solid golf ball of the invention can be manufacturedusing an ordinary process such as a known injection molding process toform on top of one another the respective layers described above: thecore, the envelope layer, the intermediate layer, and the cover. Forexample, a molded and vulcanized article composed primarily of a rubbermaterial may be placed as the core within a particular injection-moldingmold, following which the envelope layer-forming material and theintermediate layer-forming material may be injection-molded in thisorder over the core to give an intermediate spherical body. Thespherical body may then be placed within another injection-molding moldand the cover material injection-molded over the spherical body to givea multi-piece golf ball. Alternatively, the cover may be formed as alayer over the intermediate spherical body by, for example, placing twohalf-cups, molded beforehand as hemispherical shells, around theintermediate spherical body so as to encase it, then molding underapplied heat and pressure.

The inventive golf ball has a surface hardness (also referred to as the“cover surface hardness”) which is determined by the hardnesses of thematerials used in each layer, the hardnesses of the respective layers,and the hardness below the surface of the ball. The surface hardness ofthe ball, expressed as the JIS-C hardness, is preferably at least 83,more preferably at least 86, and even more preferably at least 88, butpreferably not more than 100, more preferably not more than 97, and evenmore preferably not more than 94. If this hardness is lower than theabove range, the ball may be too receptive to spin, as a result of whichan increased distance may not be achieved. On the other hand, if thesurface hardness of the ball is higher than the above range, the ballmay not be receptive to spin on approach shots, which may result in aless than desirable controllability even for professionals and otherskilled golfers.

It is desirable for the surface hardness of the inventive golf ball tobe made softer than the surface hardness of the intermediate layer by anamount, expressed in JIS-C hardness units, of preferably at least 1,more preferably at least 2, and even more preferably at least 3, butpreferably not more than 10, more preferably not more than 8, and evenmore preferably not more than 6. At a hardness difference smaller thanthis range, the ball may lack receptivity to spin on approach shots,resulting in a less than desirable controllability even for professionaland other skilled golfers. At a hardness difference larger than theabove range, the rebound may be inadequate or the ball may be tooreceptive to spin on full shots, as a result of which the desireddistance may not be achieved.

Numerous dimples may be formed on the surface of the cover. The dimplesarranged on the cover surface, while not subject to any particularlimitation, number preferably at least 280, more preferably at least300, and even more preferably at least 320, but preferably not more than360, more preferably not more than 350, and even more preferably notmore than 340. If the number of dimples is higher than the above range,the ball will tend to have a low trajectory, which may shorten thedistance of travel. On the other hand, if the number of dimples is toosmall, the ball will tend to have a high trajectory, as a result ofwhich an increased distance may not be achieved.

Any one or combination of two or more dimple shapes, including circularshapes, various polygonal shapes, dewdrop shapes and oval shapes, may besuitably used. If circular dimples are used, the diameter of the dimplesmay be set to at least about 2.5 mm but not more than about 6.5 mm, andthe depth may be set to at least 0.08 mm but not more than 0.30 mm.

To fully manifest the aerodynamic characteristics of the dimples, thedimple coverage on the spherical surface of the golf ball, which is thesum of the individual dimple surface areas, each defined by the borderof the flat plane circumscribed by the edge of a dimple, expressed as aratio (SR) with respect to the spherical surface area of the ball wereit to be free of dimples, is preferably at least 60% but not more than90%. Also, to optimize the trajectory of the ball, the value V₀ obtainedby dividing the spatial volume of each dimple below the flat planecircumscribed by the edge of that dimple by the volume of a cylinderwhose base is the flat plane and whose height is the maximum depth ofthe dimple from the base is preferably at least 0.35 but not more than0.80. In addition, the VR value, which is the sum of the volumes of theindividual dimples formed below the flat plane circumscribed by the edgeof the respective dimple, as a percentage of the volume of the ballsphere were it to have no dimples thereon, is preferably at least 0.6%but not more than 1.0%. Outside the above ranges for these values, theball may assume a trajectory that is not conducive to achieving a gooddistance, as a result of which the ball may fail to travel a sufficientdistance when played.

The golf ball of the invention, which can be manufactured so as toconform with the Rules of Golf for competitive play, may be produced toa ball diameter which is of a size that will not pass through a ringhaving an inside diameter of 42.672 mm, but is not more than 42.80 mm,and to a weight of generally from 45.0 to 45.93 g.

As shown above, by having the core made of two layers—an inner layer andan outer layer—which are each formed primarily of a rubber material insuch a way that the outer core layer is harder than the inner corelayer, and by optimizing the respective thicknesses and hardnesses ofthe envelope layer, the intermediate layer and the cover as describedabove, the inventive golf ball having a multi-layer construction ishighly beneficial for professionals and other skilled golfers because itlowers the spin rate on full shots with a driver, providing increaseddistance and good controllability, especially the ability to maintain astraight trajectory on full shots, and also has an excellent scuffresistance.

EXAMPLES

Examples of the invention and Comparative Examples are given below byway of illustration, and not by way of limitation.

Examples 1 and 2, Comparative Examples 1 to 5 Formation of Core

Rubber compositions were formulated as shown in Tables 1 and 2, thenmolded and vulcanized at 155° C. for 15 minutes to form an inner corelayer and an outer core layer. That is, the rubber composition for aninner core layer shown in Table 1 was prepared and vulcanized, followingwhich the resulting inner core layer was enveloped by an outer corelayer made of the material shown in Table 2 in an unvulcanized state,and the resulting sphere was molded and vulcanized to give a two-layerconstruction.

TABLE 1 Example Comparative Example Rubber formulation 1 2 1 2 3 4 5Inner Polybutadiene 100 100 100 100 100 100 100 core layer Zinc acrylate17 22 34.5 26.7 22 22 22 formulation Peroxide 1.2 1.2 1.2 1.2 1.2 1.21.2 Antioxidant 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Zinc oxide 34.6 32.8 27.6324 32.8 30.1 32.8 Zinc salt of 2.5 2.5 1 1 2.5 2.5 2.5pentachlorothiophenol Zinc stearate 5 5 0 5 5 5 5 VulcanizationTemperature (° C.) 155 155 155 155 155 155 155 Time (min) 15 15 15 15 1515 15

TABLE 2 Example Comparative Example Rubber formulation 1 2 1 2 3 4 5Outer core Polybutadiene 100 100 — 100 100 100 100 layer Zinc acrylate35 35 — 35 35 35 35 formulation Peroxide 1.2 1.2 — 1.2 1.2 1.2 1.2Antioxidant 0.1 0.1 — 0.1 0.1 0.1 0.1 Zinc oxide 28.3 28.3 — 20.1 28.325.4 28.3 Zinc salt of 2 2 — 2 2 2 2 pentachlorothiophenol Zinc stearate5 5 — 5 5 5 5 Vulcanization Temperature (° C.) 155 155 — 155 155 155 155Time (min) 15 15 — 15 15 15 15

Trade names for key materials appearing in the tables are given below.Numbers in the tables represent parts by weight.

-   Polybutadiene: Available from JSR Corporation under the trade name    BR 730.-   Peroxide: A mixture of 1,1-di(t-butylperoxy)cyclohexane and silica,    produced by NOF Corporation under the trade name Perhexa C-40.-   Antioxidant: 2,2′-Methylenebis(4-methyl-6-t-butylphenol), produced    by Ouchi Shinko Chemical Industry Co., Ltd. under the trade name    Nocrac NS-6.-   Zinc stearate: Available from NOF Corporation under the trade name    Zinc Stearate G.

[Formation of Envelope Layer, Intermediate Layer and Cover]

Next, envelope layer, intermediate layer and cover formulations of thevarious resin ingredients shown in Table 3 were injection-molded overthe two-layer core so as to form, in order: an envelope layer, anintermediate layer and a cover. Finally, the dimples shown in Table 4and FIG. 2, which were common to all the examples, were formed on thecover surface, thereby producing multi-piece solid golf balls.

TABLE 3 Formulation (pbw) No. 1 No. 2 No. 3 No. 4 No. 5 Himilan 1605 5068.75 Himilan 1557 15 Himilan 1706 35 Surlyn 8120 75 Dynaron 6100P 2531.25 Hytrel 4001 15 Behenic acid 20 18 Calcium hydroxide 2.3 2.3Calcium stearate 0.15 0.15 Zinc stearate 0.15 0.15 Trimethylolpropane1.1 Polytail H 2 Pandex T-8290 100 Pandex T-8260 100 Titanium oxide 3.53.8 Polyethylene wax 1.5 1.4 Isocyanate compound 9 Isocyanate mixture 18

Trade names for key materials appearing in the table are given below.

-   Himilan: Ionomer resins produced by DuPont-Mitsui Polychemicals Co.,    Ltd.-   Surlyn: An ionomer produced by E.I. DuPont de Nemours & Co.-   Dynaron 6100P: A hydrogenated polymer produced by JSR Corporation.-   Hytrel 4001: A polyester elastomer produced by DuPont-Toray Co.,    Ltd.-   Behenic acid: NAA222-S (beads), produced by NOF Corporation.-   Calcium hydroxide: CLS-B, produced by Shiraishi Kogyo.-   Polytail H: A low-molecular-weight polyolefin polyol produced by    Mitsubishi Chemical Corporation.-   Pandex T-8260, T-8290: MDI-PTMG type thermoplastic polyurethanes    produced by DIC Bayer Polymer.-   Polyethylene wax: Produced by Sanyo Chemical Industries, Ltd. under    the trade name Sanwax 161P.-   Isocyanate compound: 4,4′-Diphenylmethane diisocyanate. The    isocyanate compound was mixed with Pandex at the time of injection    molding.-   Isocyanate mixture: An isocyanate master batch produced by Dainichi    Seika Colour & Chemicals Mfg. Co., Ltd. under the trade name    Crossnate EM30. Contains 30% of 4,4′-diphenylmethane diisocyanate    (measured concentration of amine reverse-titrated isocyanate    according to JIS-K1556, 5 to 10%). A polyester elastomer was used as    the master batch base resin.

TABLE 4 Number of Diameter Depth No. dimples (mm) (mm) V₀ SR VR 1 12 4.60.15 0.47 0.81 0.783 2 234 4.4 0.15 0.47 3 60 3.8 0.14 0.47 4 6 3.5 0.130.46 5 6 3.4 0.13 0.46 6 12 2.6 0.10 0.46 Total 330

[Dimple Definitions]

-   Diameter: Diameter of flat plane circumscribed by edge of dimple.-   Depth: Maximum depth of dimple from flat plane circumscribed by edge    of dimple.-   V₀: Spatial volume of dimple below flat plane circumscribed by    dimple edge, divided by volume of cylinder whose base is the flat    plane and whose height is the maximum depth of dimple from the base.-   SR: Sum of individual dimple surface areas, each defined by the    border of the flat plane circumscribed by the edge of a dimple, as a    percentage of surface area of ball sphere were it to have no dimples    thereon.-   VR: Sum of volumes of individual dimples formed below flat plane    circumscribed by the edge of the dimple, as a percentage of volume    of ball sphere were it to have no dimples thereon.

The golf balls obtained in Examples 1 and 2 of the invention and inComparative Examples 1 to 5 were tested and evaluated according to thecriteria described below with regard to the following: surface hardnessand other physical properties of each layer and the ball, flightperformance (on shots with a driver and shots with an iron), spin onapproach shots (controllability), and scuff resistance. The results areshown in Tables 5 and 6. All measurements were carried out in a 23° C.atmosphere.

(1) Core Deflection

The core was placed on a hard plate, and the deflection (mm) by the corewhen compressed under a final load of 1,275 N (130 kgf) from an initialload of 98 N (10 kgf) was measured.

(2) Core Surface Hardness

The durometer indenter was set substantially perpendicular to thespherical surface of the core, and JIS-C hardness measurements (inaccordance with JIS-K6301) were taken at two randomly selected points onthe core surface. The average of the two measurements was used as thecore surface hardness.

(3) Hardness of Envelope Layer Material

The resin material for the envelope layer was formed into a sheet havinga thickness of about 2 mm, and the hardness of the material was measuredwith a type D durometer in accordance with ASTM D-2240.

(4) Surface Hardness of Envelope Layer-Covered Sphere

The durometer indenter was set substantially perpendicular to thespherical surface of the envelope layer, and the JIS-C hardness wasmeasured.

(5) Hardness of Intermediate Layer Material

The same method of measurement was used as in (3) above.

(6) Surface Hardness of Intermediate Layer-Covered Sphere

The durometer indenter was set substantially perpendicular to thespherical surface of the intermediate layer and the JIS-C hardness wasmeasured.

(7) Hardness of Cover Material

The same method of measurement was used as in (3) above.

(8) Surface Hardness of Ball

The durometer indenter was set substantially perpendicular to adimple-free area on the ball's surface and the JIS-C hardness wasmeasured.

(9) Flight Performance on Shots with Driver

The carry and total distance of the ball when hit at a head speed (HS)of 45 m/s with a driver (TourStage X-Drive 410 (2007 model),manufactured by Bridgestone Sports Co., Ltd.; loft angle, 9.5°) mountedon a swing robot were measured. The results were rated according to thecriteria shown below. The spin rate was the value measured for the ballimmediately following impact, using an apparatus for measuring initialconditions.

Good: Total distance was 235 m or more

NG: Total distance was less than 235 m

(10) Flight Performance on Shots with Iron

The carry and total distance of the ball when hit at a head speed (HS)of 45 m/s with an iron (abbreviated below as “I#6”; TourStage X-Blade(2005 model), manufactured by Bridgestone Sports Co., Ltd.) mounted on aswing robot were measured. The results were rated according to thecriteria shown below. The spin rate was measured in the same way asdescribed above.

Good: Total distance was 175 m or more

NG: Total distance was less than 175 m

(11) Spin Rate on Approach Shots

The spin rate of a ball hit at a head speed of 22 m/s with a sand wedge(abbreviated below as “SW”; J's Classical Edition, manufactured byBridgestone Sports Co., Ltd.) was measured. The results were ratedaccording to the criteria shown below. The spin rate was measured by thesame method as that used above when measuring distance.

Good: Spin rate of 6,000 rpm or more

NG: Spin rate of less than 6,000 rpm

(12) Scuff Resistance

A non-plated pitching sand wedge was set in a swing robot, and the ballwas hit once at a head speed of 40 m/s, following which the surfacestate of the ball was visually examined and rated as follows.

Good: Can be used again

NG: Cannot be used again

TABLE 5 Example Comparative Example 1 2 1 2 3 4 5 Core Inner Diameter(mm) 21.95 21.95 34.95 21.95 21.95 21.95 21.95 core Weight (g) 6.79 6.7927.38 6.53 6.79 6.70 6.65 layer Deflection (mm) 6.8 6.0 3.5 6.0 6.0 6.06.0 Surface JIS-C 60 68 83 68 68 68 68 hardness Shore D 38 44 55 44 4444 44 Center JIS-C 50 55 64 55 55 55 55 hardness Shore D 30 34 40 34 3434 34 Outer core Thickness (mm) 6.6 6.6 — 7.7 6.6 6.6 7.2 layer 2-layerDiameter (mm) 35.15 35.18 34.95 37.25 35.18 35.18 36.3 core Weight (g)27.88 27.94 27.38 31.93 27.94 27.58 30.05 (inner Deflection (mm) 4.2 3.83.5 3.6 3.8 3.8 3.7 layer + Surface JIS-C 84 84 83 84 84 84 84 outerhardness Shore D 56 56 55 56 56 56 56 layer) (Outer core layer JIS-C 3529 19 29 29 29 29 surface) − (Inner Shore D 26 22 15 22 22 22 22 corelayer surface) (Deflection by overall core)/ 0.62 0.63 — 0.60 0.63 0.630.62 (Deflection by inner core layer) Envelope Material No. 1 No. 1 No.1 — No. 1 No. 1 No. 1 layer Material hardness (Shore D) 51 51 51 — 51 5151 Thickness (mm) 1.55 1.56 1.70 — 1.56 1.56 1.00 Specific gravity 0.9450.945 0.945 — 0.945 0.945 0.945 Envelope Diameter (mm) 38.26 38.30 38.35— 38.30 38.30 38.30 layer- Weight (g) 34.10 34.21 34.17 — 34.21 33.8534.19 encased Deflection (mm) 3.58 3.38 3.15 — 3.38 3.38 3.38 sphereInter- Material No. 2 No. 2 No. 2 No. 2 No. 5 No. 2 No. 2 mediateMaterial hardness (Shore D) 62 62 62 62 56 62 62 layer Thickness (mm)1.19 1.17 1.15 1.70 1.17 0.85 1.17 Specific gravity 0.95 0.95 0.95 0.950.93 0.95 0.95 Inter-mediate Diameter (mm) 40.65 40.64 40.65 40.65 40.6440.00 40.64 layer-encased Weight (g) 39.65 39.65 39.52 39.64 39.54 37.7339.64 sphere Cover Material No. 3 No. 3 No. 3 No. 3 No. 4 No. 3 No. 3Thickness (mm) 1.02 1.03 1.03 1.03 1.03 1.35 1.03 Specific gravity 1.151.15 1.15 1.15 1.15 1.15 1.15 Material hardness (Shore D) 49 49 49 49 5849 49 Ball Diameter (mm) 42.7 42.7 42.7 42.7 42.7 42.7 42.7 Weight (g)45.47 45.49 45.34 45.46 45.37 45.46 45.47

TABLE 6 Example Comparative Example 1 2 1 2 3 4 5 Flight W#1 Spin rate(rpm) 2578 2676 2725 2735 2615 2755 2785 (HS, Carry (m) 211.5 213.6214.4 212.5 2120 211.1 212.7 45 m/s) Total distance (m) 236.1 235.6236.5 233.7 235.1 231.5 233.6 Rating Good Good Good NG Good NG NG I#6Spin rate (rpm) 5949 6180 6533 6255 6155 6345 6222 Carry (m) 165.2 165.9164.9 165 163.3 165.0 164.8 Total distance (m) 175.4 178.3 173.6 175.9175.5 173.9 175.5 Rating Good Good NG Good Good NG Good SW Spin rate(rpm) 6293 6421 6381 6445 5785 6475 6397 (HS, Rating Good Good Good GoodNG Good Good 22 m/s) Scuff resistance Good Good Good Good NG Good Good

As is apparent from the results in Table 6, in Comparative Example 1,because the core had only one layer, the spin rate-lowering effect onshots with an iron (I#6) was inadequate and a satisfactory distance wasnot achieved. In Comparative Example 2, because the golf ball lacked anenvelope layer, the spin rate-lowering effect on shots with a driver(W#1) was inadequate and a satisfactory distance was not achieved. InComparative Example 3, because the cover (outermost layer) was hard, theball lacked sufficient spin on approach shots and the scuff resistancewas poor. In Comparative Example 4, the cover was formed so as to bethicker than the intermediate layer, resulting in an increase in thespin rate of the ball and a decrease in rebound, and thus a less thansatisfactory distance. In Comparative Example 5, the envelope layer wasformed so as to be thinner than the intermediate layer, resulting in aninsufficient spin rate-lowering effect on shots taken with a W#1 andthus a less than satisfactory distance.

1. A multi-piece solid golf ball comprising a core, an envelope layerencasing the core, an intermediate layer encasing the envelope layer,and a cover which encases the intermediate layer and has formed on asurface thereof a plurality of dimples, wherein the envelope layer hasan inner layer and an outer layer; the core is formed primarily of arubber material; the inner and outer envelope layers, the 10intermediate layer and the cover are each formed primarily of the sameor different resin materials; the inner and outer envelope layers,intermediate layer and cover have thicknesses which satisfy thefollowing conditions (i) and (ii): (i) cover thickness > intermediatelayer thickness > (inner envelope layer thickness + outer envelope layerthickness) (ii) (cover thickness + intermediate layer thickness) >(innerenvelope layer thickness + outer envelope layer thickness); and the coresurface, inner envelope layer, outer envelope layer, intermediate layerand cover have Shore D hardnesses which satisfy the following condition(iii): (iii) core surface hardness ≦ inner envelope layer hardness <outer envelope layer hardness < intermediate layer hardness > coverhardness.
 2. The multi-piece solid golf ball of claim 1, wherein theresin material of the inner envelope layer and/or the outer envelopelayer comprises, in admixture, an ionomer resin component of (a) anolefin-unsaturated carboxylic acid random copolymer and/or a metal ionneutralization product of an olefin-unsaturated carboxylic acid randomcopolymer mixed with (b) an olefin-unsaturated carboxylicacid-unsaturated carboxylic acid ester random terpolymer and/or a metalion neutralization product of an olefin-unsaturated carboxylicacid-unsaturated carboxylic acid ester random terpolymer in a weightratio between 100:0 and 0:100, and (e) a non-ionomeric thermoplasticelastomer in a weight ratio between 100:0 and 50:50.
 3. The multi-piecesolid golf ball of claim 1, wherein the resin material of the innerenvelope layer and/or the outer envelope layer is a mixture comprising:100 parts by weight of a resin component composed of, in admixture, abase resin of (a) an olefin-unsaturated carboxylic acid random copolymerand/or a metal ion neutralization product of an olefin-unsaturatedcarboxylic acid random copolymer mixed with (b) an olefin-unsaturatedcarboxylic acid-unsaturated carboxylic acid ester random terpolymerand/or a metal ion neutralization product of an olefin-unsaturatedcarboxylic acid-unsaturated carboxylic acid ester random terpolymer in aweight ratio between 100:0 and 0:100, and (e) a non-ionomericthermoplastic elastomer in a weight ratio between 100:0 and 50:50; (c) 5to 80 parts by weight of a fatty acid and/or fatty acid derivativehaving a molecular weight of 228 to 1500; and (d) 0.1 to 17 parts byweight of a basic inorganic metal compound capable of neutralizingun-neutralized acid groups in the base resin and component (c).
 4. Themulti-piece solid golf ball of claim 1, wherein the envelope layerthickness is at least twice the intermediate layer thickness.
 5. Themulti-piece solid golf ball of claim 1, wherein the core has adeflection when compressed under a final load of 1,275 N (130 kgf) froman initial load of 98 N (10 kgf) of at least 3.6 mm but not more than12.0 mm, and the ball as a whole has a deflection when compressed undera final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf)of at least 1.8 mm but not more than 4.0 mm.
 6. The multi-piece solidgolf ball of claim 1, wherein the core has a deflection (P) whencompressed under a final load of 1,275 N (130 kgf) from an initial loadof 98 N (10 kgf) and the ball as a whole has a deflection (Q) whencompressed under a final load of 1,275 N (130 kgf) from an initial loadof 98 N (10 kgf) such that the value (P)-(Q) is at least 1.8 mm but notmore than 10.0 mm.
 7. The multi-piece solid golf ball of claim 1,wherein the cover is formed by injection molding a single resin blendcomposed primarily of (A) a thermoplastic polyurethane and (B) apolyisocyanate compound, which resin blend contains a polyisocyanatecompound in at least some portion of which all the isocyanate groupsremain in an unreacted state.